This invention relates generally to nuclear magnetic resonance imaging (MRI), and more particularly the invention relates to motion analysis and imaging of an organ such as the heart using phase contrast MRI maps of tissue velocity in the organ.
Assessment of the motion of the heart muscle, myocardial motion, is fundamental to the characterization of certain cardiac pathologies and to the development and evaluation of successful interventions.
Echocardiography has become the preferred method for clinical studies of myocardial dynamics, providing local measures of both wall motion and thickening. The basic shortcoming of echocardiography and other tomographic techniques including CT and conventional MRI is that they use a fixed external reference system and are unable to track an individual segment of myocardium. With these methods, the myocardium present in a fixed imaging plane is imaged. Thus, the myocardial tissue seen within this imaging plane may actually represent different samples of tissue throughout the cardiac cycle. One result is that the measure wall motion and thickening may be in error. The accurate identification of specific points fixed in the myocardium provides an appropriate solution to this limitation. One method capable of providing this information uses implanted radiopaque markers but is very invasive.
Magnetic resonance imaging (MRI) is a noninvasive method which has been shown to provide accurate measures of global myocardial function, ventricular volumes, and regional wall thickening. MRI can provide similar information to echocardiography in virtually any imaging plane as it does not suffer the limitation of requiring an acoustic window. In addition, MRI provides full field anatomical images and is less dependent on operator skill. One MRI method that has been found to be very useful in studies of the heart is called CINE MRI, and is described in U.S. Pat. No. 4,710,717. Briefly, with this method, data are acquired at rapid rates and the incrementation of the amplitude of the phase encoding gradient is controlled using a physiological trigger, e.g. EKG. While this occurs, the temporal position within the cardiac cycle at which each echo was acquired is also measured. Using this timing information and interpolation methods, images that portray the appearance of the object throughout the cardiac cycle can be formed. While useful for cardiac studies, these images still suffer from the disadvantages described above.
MRI images typically have an image intensity dependent on spin density and relaxation effects. However, methods that produce images whose intensity is proportional to velocity have also been demonstrated. See, for example, O'Donnell, Med. Physics, 12: 59-64, 1985; Spirtzer Radiology, 176: 255-262, 1990, and Nayler et al., J. Computer Assisted Tomography, 10: 715-722, 1986. These methods generally belong to the class of methods called phase contrast MRI. Copending U.S. patent application Ser. No. 07/564,945, filed Aug. 9, 1990, for Encoding For NMR Phase Contrast Flow Measurement, discloses a particularly useful and efficient method for simultaneously measuring the three components of velocity, as well as an apparatus with which the method can be performed. Phase contrast principles have been combined with the cine imaging method described above to enable the production of images that portray the distribution of velocities at multiple points in the cardiac cycle.
The recently introduced myocardial tagging method with MRI, however, offers a noninvasive technique for obtaining information about the motion of specific myocardial sites similar to that derived from implanted markers. See Zerhouni et al., "Human Heart: Tagging with MR Imaging--A method for Noninvasive Assessment of Myocardial Motion", Radiology 1988; 169:59-63, and Axel, "MR Imaging of Motion with Spatial Modulation of Magnetization", Radiology 1989; 171:841-845.
However, there are several shortcomings to the current implementations of the MRI based myocardial tagging techniques. Their use with spin echo imaging intrinsically limits temporal resolution and extends exam time such that true 3D data is difficult to obtain. The thickness, number and spacing of "sat bands" limits the spatial resolution of myocardial motion analyses. The resolution of motion analysis is further limited by the spatial resolution of the images. Methods of visualizing motion in all three dimensions have yet to be demonstrated. Analysis of these data require sophisticated pattern recognition algorithms or manual intervention. Finally, the contrast of the tags decays due to T.sub.1 relaxation, thereby making examination of the entire cardiac cycle difficult.
Wedeen (Proc. of Soc. Mag. Res. in Med., August 1990, p. 462) has shown that parameters related to strain can be derived from MRI maps of velocity. However, only a single time frame is treated and no teaching is presented as to how multiple time frames may be combined. Further, the paper does not recognize the presence of rigid rotation nor exploits information about translations.